1. Field of the Invention
The present invention relates to a digital tuner for receiving a digital signal.
2. Description of Related Art
As an example of a conventional digital tuner, a digital tuner for receiving a television broadcast will be described below. FIG. 15 shows an example of the configuration of a conventional digital tuner. The conventional digital tuner shown in FIG. 15 is provided with an RF input terminal 1, a band-pass filter (BPF) 2, an RF amplifier 3, a band-pass filter (BPF) 4, a tuning voltage generator circuit 5, a voltage-controlled oscillator (VCO) 6, a mixer 7, and an IF output terminal 9.
A number of reception signals are fed to the RF input terminal 1, and, of these reception signals, only the desired reception signal is selected by the band-pass filter 2, of which the filtering characteristics vary according to a tuning voltage outputted from the tuning voltage generator circuit 5. The desired reception signal selected by the band-pass filter 2 is then amplified by the RF amplifier 3. Thereafter, of the signals outputted from the RF amplifier 3, only the desired reception signal is further selected by the band-pass filter 4, of which the filtering characteristics vary according to the tuning voltage outputted from the tuning voltage generator circuit 5. The thus selected desired reception signal is then fed to the mixer 7. The mixer 7 also receives a local oscillation signal oscillated by the voltage-controlled oscillator 6. Here, the frequency of the local oscillation signal varies according to the tuning voltage outputted from the tuning voltage generator circuit 5. The mixer 7 performs frequency conversion by mixing the desired reception signal with the local oscillation signal so that the desired reception signal is down-converted into an intermediate-frequency (IF) signal, which is then outputted to the IF output terminal 9. The frequency of the intermediate-frequency signal is determined by the difference between the frequency of the local oscillation signal and that of the desired reception signal. The tuning voltage generator circuit 5 generates different tuning voltages for different reception channels. Thus, the frequency of the local oscillation signal is varied in proportion to that of the desired reception signal, so that the frequency of the intermediate-frequency signal outputted from the mixer 7 remains constant.
In this way, the conventional digital tuner shown in FIG. 15 selects one from a number of reception signals transmitted from different television broadcasting stations. The intermediate-frequency signal outputted from the IF output terminal 9 is fed to a digital signal demodulation circuit (not illustrated) so as to be demodulated.
The reception signals fed to the RF input terminal 1 are each a radio-frequency carrier wave having superimposed thereon a signal obtained by modulating digital data by a modulation method such as OFDM, QAM, QPSK, 8VSB, or CDMA. As shown in FIG. 16, such a reception signal has an approximately constant level over largely the entire reception band, and thus exhibits a waveform without inclination.
However, the conventional digital tuner shown in FIG. 15 has the following drawbacks. Even when a reception signal having an approximately constant level over largely the entire reception band is fed to it, it cannot make approximately constant the level of the intermediate-frequency signal outputted from the IF output terminal. This is due to, in particular, variations in the filtering characteristics of the band-pass filters 2 and 4, and causes the outputted intermediate-frequency signal to have a waveform with an inclination as shown in FIG. 17. When the intermediate-frequency signal outputted from the IF output terminal has an inclined waveform, the digital signal demodulation circuit produces many errors, and in the worst case cannot output an image, sound, or data signal.
With the conventional digital tuner shown in FIG. 15, it is impossible to completely eliminate the waveform inclination of the intermediate-frequency signal, and it is only possible to alleviate it by reducing variations in the filtering characteristics through adjustment of the filtering characteristics of the band-pass filters 2 and 4.
Moreover, the conventional digital tuner shown in FIG. 15 is susceptible to signal deterioration within the reception band when interfered from outside the reception band.